In recent years against the background of development to a large-scale integration of semiconductor integrated circuits and progress to a larger capacity of recording media, a higher cleanliness level of wafers employed for fabrication of them has been required. This is because such wafers produce causes for generation of pattern defects of semiconductor integrated circuits and recording errors of recording media only if a small number of particles exist on surfaces thereof. Therefore, while the wafers are required to be kept in a high cleanliness level, cases storing the wafers are inevitably requested to be in a high cleanliness level. When a wafer storage case storing wafers is contaminated, the wafers in the wafer storage case eventually cannot be of a high cleanliness level even if the wafers are cleaned to a high cleanliness level.
As a wafer storage case, for example, there has been known such construction as shown in FIGS. 9 to 11. In the figures, a wafer storage case 12 is constituted of a case body 14 housing wafers; and a lid 16 closing an upper opening section of the case body 14. There is mounted in the case body 14 a substrate-housing cassette 18 in which many wafers W are housed, as shown in FIG. 11. Note that a reference numeral 20 is a packing to be mounted along a peripheral portion of the upper opening section of the case body 14 and a reference numeral 22 is a substrate retainer mounted on the upper portion of the substrate-housing cassette 18.
Synthetic resins such as polypropylene and polycarbonate are generally used as materials of a wafer storage case from the viewpoint of preventing contamination by dust particles and chemicals, convenience of handling and cost. The wafer storage case is subjected to cleaning for acquiring a high cleanliness level. There are conventionally known various kinds of cleaning methods for the purpose and among them, wet cleaning is generally employed; to be detailed, there can be named ultrasonic cleaning with ultrasonic waves, shower cleaning using a high water pressure, brush cleaning in which a case is rubbed with a brush and other cleaning methods.
In various ways of semiconductor cleaning such as cleaning of the case and a wafer itself, there is used pure water or ultrapure water of specific resistance (or resistivity) of 10 MΩ·cm or higher which is obtained by removing fine particles, organic materials, dissolved gases in water such as dissolved oxygen and carbon dioxide gas, inorganic ions and others. Especially, a quality level of pure or ultrapure water used in fabrication of a semiconductor device has become further higher with increase in integration thereof and is currently almost close to the theoretical pure water value of 18.24 MΩ·cm, where no impurity is contained. In an ordinary wafer fabrication process, pure or ultrapure water of about 16 MΩ·cm or higher is used in cleaning not only wafers but also storage cases and others in which a high cleanliness level is required.
There are used as cleaning liquids, not to speak of a pure water, a surfactant, an organic solvent, acid and others, and cleaning of a synthetic resin storage case has been performed in combination of the cleaning liquids and the above-described cleaning methods. A cleaning method using a surfactant, which is typical of a cleaning method for a wafer storage case 12, is performed, as shown in FIG. 12, in a procedure of surfactant cleaning (a)→pure water cleaning (with water subjected to ion exchange treatment) (b)→pure water cleaning (with ultrapure water) (c)→a clean oven drying (d).
Along with more vigorous requirements for a cleanliness level of a wafer in recent years, requirements for a cleanliness level of a storage case for storing and transporting wafers have also become severer and the above-mentioned prior art cleaning methods have been approaching their limits in cleaning capability. That is, even if a storage case is cleaned by means of the cleaning methods using ultrapure cleaning liquid, there has arisen a problem that particles increase on wafers housed in a storage case during transportation. While this would considered to be caused by dust generation from housed wafers themselves, wafers now each having ultrahigh precision are of a nature very low in dust generation from themselves but poor cleaning of a storage case and a storage case itself serve as main causes for dust generation.
The above-mentioned causes have not been made clear heretofore, but, for example, the following explanation is given: In the prior art cleaning methods, a storage case is of synthetic resin whose surface is hydrophobic and has so-called poor wettability to avoid contact with water. For this reason, even with physical energy such as ultrasonic waves applied, a physical action thereof is not sufficiently transferred to surfaces of the synthetic resin and particles thereon so that efficient particle removal cleaning is not performed presumably, thereby leading to poor cleaning by guess.
Dust generation subsequent to the cleaning is considered to be due to particles originating from fuzzes or protrusions in micrometers residing on a synthetic resin surface of a wafer storage case and the particles are released from the synthetic resin surface with the lapse of time due to vibration of the wafer storage case, rubbing of the case against wafers housed therein or the like during transportation thereof, thus released dust particles adhering to the synthetic resin surface. Furthermore, it is considered that increase in complexity of a shape of a storage case in recent years accelerates poor cleaning of the storage case and dust generation from the storage case itself (see JP-A-96-59863).
The inventor of the present invention has conducted researches on a relationship between a prior art cleaning method and dust generation from surfaces of a wafer storage case made of synthetic resin with the result that it has been found that no matter how much high purity cleaning is repeatedly applied to the wafer storage case, particles are constantly generated from the surfaces of synthetic resin since the surfaces of synthetic resin is porous as shown in FIG. 13. Description will be given of states shown schematically in FIGS. 14 (a)˜(c) of a synthetic resin surface and particles in connection with particle generation (dust generation) in the case where a wafer is cleaned by means of a prior art cleaning method using a surfactant shown in FIG. 12.
Many particles P exist on a porous synthetic resin surface 12a prior to cleaning as described above (FIG. 13). When the synthetic resin surface 12a is cleaned with a surfactant (for example, 0.3% aqueous solution), the particles P are separated from the surface 12a to move into a cleaning liquid, and after the cleaning, a surfactant layer L attached to the surface 12a is formed in a state where the surfactant layer L blocks newly generated particles P on the synthetic resin surface 12a (FIGS. 12(a) and 14(a)).
Free particles P which are separated from the synthetic resin surface 12a during the cleaning, moved into the surfactant aqueous solution and floating therein are again adsorbed or adhered to a surface of the surfactant layer L. At this time, while particles P originally existing on the synthetic resin surface 12a are separated by the cleaning to move into the cleaning liquid, since new particles are generated in the vicinity of the synthetic resin surface 12a or from the interior thereof in a porous state, particles P still exist on the synthetic resin surface 12a as shown in the schematic figure (FIG. 14(a)). The particles from the interior in the porous state are considered to cause new dust generation from a storage case itself due to poor cleaning and vibration.
When the synthetic resin surface 12a cleaned with the surfactant is further cleaned with pure water, the surfactant layer L formed on the synthetic resin layer 12a is washed; therefore, the particles P on the surfactant layer L are washed away, and the surfactant layer L is also partly washed away to partly remain on the synthetic resin surface 12a (FIGS. 12(b) and 14(b)). At this time, since the surfactant layer L is washed away to expose a bare surface 12a, the particles P blocked by the surfactant layer L is separated from the exposed surface 12a during the cleaning to move into the pure water cleaning liquid. Also, new particles P are generated on the bare surface 12a from which particles have been separated so that the particles P exist on the synthetic resin surface 12a in a similar manner as shown in the schematic figure (FIG. 14(b)).
When the cleaning with pure water is repeated, the surfactant layer L partly left on the synthetic resin surface 12a is further washed away by pure water; therefore, the surfactant layer L remains in a further shrunken state on the synthetic resin surface 12a (FIGS. 12(c) and 14(c)). At this time, particles P blocked by the remaining surfactant layer L further move into the pure water cleaning liquid since the surfactant layer L is further washed away to increase a bare surface 12a. As described above, new particles P are generated on the bare surface 12a from which particles have been separated, and thus many particles P exist on the synthetic resin surface 12a in a similar manner (FIG. 14(c)).
At the last stage, the wafer storage case 12 after cleaning is dried as it is (for example, by clean oven drying) (FIGS. 12(d) and 14(d)). Therefore, many particles P exist on the synthetic resin surface 12a in an unbound state, so the particles P are separated with ease from the synthetic resin surface due to vibration and rubbing during transportation, which causes dust generation.
Thus, the inventor has confirmed that the prior art cleaning methods, as described above, which were performed for prevention of dust generation on a synthetic resin wafer storage case have almost no effect of prevention of dust generation. Taking into consideration that since such prior art dust generation preventing measures overlook constant particle generation from the synthetic resin surface, dust generation cannot be prevented no matter how highly accurately the cleaning may be performed and hence it is impossible to reach a fundamental solution, the inventor has conducted earnest researches in order to propose a perfect solution for prevention of dust generation. As a result, the inventor has completed the present invention on the basis of a reverse conception where there is not used the cleaning technique to which the prior art methods stick.